Actually, the problem is that the premise is false: Thor's powers *haven't* been adequate for dealing with Kurse. Traditionally, Thor "deals" with Kurse by the means of "fight him for a while, get the crap kicked out of him, then plot device happens." The only esoteric power Thor has that could really help versus Kurse is dumping him in another dimension, which runs into the problem of "creating a dimensional rift takes time and concentration, meanwhile, Kurse stomps Thor's face into the ground."